The present invention relates to upgraded nylon fibers and to reinforced cementitious composites including same. More particularly the present invention relates to the use of non-aromatic polyamide (nylon) fibers or filaments (both are referred herein as fibers) in the production of cementitious composites, such as, but not limited to, types of concretes, shotcretes, mortars and the like. Most particularly the present invention relates to a method of upgrading the strength and durability of nylon fibers, such as textile nylon fibers, to render same suitable for use in reinforcing the cementitious matrix of cementitious composites, such as concrete.
As is well known in the art, cementitious composites, such as concrete, are prone to self induced cracking, as such composites are brittle by nature. Self induced cracks readily propagate through concrete under relatively low stresses. Thus, concrete fails in tension by progressive crack development.
The concrete's actual low tensile strength is explained by the presence of flaws (microcracks and cracks) that propagate into bigger cracks under tension. To increase the concrete durability, it is, therefore, important to minimize the presence of microcracks and cracks that are distributed therein, which weakens the concrete and reduces its durability.
When a mix of concrete, or any other mix of a cementitious composite, is placed (e.g., poured, molded, layered, sprayed, etc.), the solids, e.g., aggregates, fines and cement, therein begin to settle downward due to gravity. As the solids sink, water is displaced and forced to the surface as bleedwater. Plastic shrinkage cracking of the concrete occurs when the rate of water evaporation exceeds the rate of water displacement. Shrinkage stresses associated with early volume change account for the majority of all non-structural cracks in concrete. As mentioned above, these cracks, which are formed while the concrete mix settles, affect the strength and durability of the concrete during service. Therefore, in the common practice, concrete products are watered and cooled while hardening. However, as watering concrete products while hardening does not completely eliminate microcracks and cracks formation and calls for special care, the search for concrete additives which reduce cracks formation has begun.
The use of nylon fibers in the reinforcement of concrete is set forth in U.S. Pat. No. 3,645,961. This patent discloses the use of discrete fibers to form a blast resistance concrete. Other related publications include U.S. Pat. Nos. 5,456,752; 5,399,195; 4,693,749; 4,902,347; and U.S. Pat. No. 1479618.
The presence of nylon fibers in a concrete mix alter the process of solids settlement and water bleeding, and therefore reduce the internal tensile stresses that lead to plastic shrinkage cracking during the early volume changes of the concrete while hardening. The stress induced microcracks that do start to form are bridged and intersected by the millions of evenly distributed fibers present in the cementitious matrix, and cracks propagation is therefore halted.
Thus, nylon fibers assist in the prevention of microcracks during settling of concrete, which microcracks form flaws which, long after settling and during service, tend to develop into bigger cracks and fractions, which weaken the concrete and reduce its durability. Nevertheless, it is important to ensure that the fibers, which constitute part of the total volume of the cementitious matrix of the concrete, will not be deteriorated during service, since the loss of internal volume strength and substance will weaken the whole concrete matrix. Thus, the requirements from nylon fibers used in concrete reinforcement are (i) efficiency in reducing microcracks formation during settling and (ii) high durability, i.e., prolonged service before deteriorating.
A substantial growth in the use of technical nylon fibers for concrete and cement reinforcement has taken place since the first trials in using nylon fibers for concrete reinforcement. Both nylon 6.6 (e.g., Du Pont Type 663 and Type 665, both are distributed by Kapejo Inc.) and nylon 6 (e.g., Alliedsignal Caprolan-RC, distributed by Nycon Inc.) prepared having technical nylon properties, are used in the art of concrete production as typical concrete secondary reinforcing fibers, aimed at combating the cracking of the concrete during the early plastic stages of its settling. The term "secondary reinforcement" is commonly used in the art of concrete production to indicate a reinforcement directed at prevention or reducing cracks associated with concrete settling.
Technical nylon fibers were the fibers of choice, as compared to textile nylon fibers, due to their higher modulus and tenacity and lower elongation. These features of technical fibers render them more suitable for concrete reinforcement and, in fact, teach away the use of textile fibers to achieve same.
Further disadvantages of textile fibers include (i) their sensitivity to ultraviolet radiation due to the presence of delustrants, typically in the form of titanium dioxide grains, which are used in textile nylon fibers to provide the fibers with a darker appearance, yet are never used in technical nylon fibers; and (ii) their high surface area to volume ratio, higher permeability due to their open and more amorphous structure and the lack of stabilizers, all increases their hydrolysis rate in the presence of high pH values, as is the case in a wet concrete mix and in a concrete product exposed to even moderate humidity levels. Therefore, textile nylon fibers were never attempted as a concrete additive.
The term "textile nylon fibers" as used in the art of nylon production and herein refers to those fibers which are typically used to form yarns for the textile industry. Such fibers, although made of an identical material differ from technical nylon fibers in many physical properties, especially thickness and strength. This is due to their intended use and the specific manufacturing procedures employed while producing textile and technical nylon fibers.
Table 1 below compares between some physical parameters of textile and technical nylon fibers, wherein the numbers in brackets indicate the typical value.
TABLE 1 ______________________________________ Technical nylon Parameter Textile nylon fibers fibers ______________________________________ Thickness 1-15 (3) Denier 5-20 (7) Denier Modulus 25-40 (35) gram/Denier 40-70 (55) gram/Denier Bending modulus low high Tenacity 3-5 (4) gram/Denier 7-11 (8) gram/Denier Elongation 35-45 (40) % *10-20 (15)% Crimp present in some fibers absent Draw ratio 3-4 (3.5) X 5-8 (6) X during manufacture Relative Viscosity* 35-45 (35) 60-100 (80) Other additives** delustrant (titanium dioxide) UV stabilizers such as Mn salts; Thermal stabilizers, such as CuI.sub.2, KI and KBr; and Anti oxidants, such as phosphorous, amino and phenolic compounds ______________________________________ *when the fibers are dissolved in 90% formic acid to form a 8.4% solution by weight, which viscosity is indicative to the number averaged molecular weight of the fibers, according to the standard convention. **added to the polymer mix before the formation of fibers during the manufacturing process.
It will, however, be appreciated that should the textile nylon fibers be modified to have higher modulus and tenacity, lower elongation, and UV and high pH resistance, such fibers will become highly suitable as a concrete reinforcement additive, since (i) their high surface area to volume ratio and therefore high specific hydrophilicity will allow them to form tighter chemical interactions with the cementitious matrix of the concrete during settling and, therefore, such fibers will function better in microcracks prevention during the settling stages; (ii) their high surface area to volume ratio and hydrophilicity provides such fibers with better water retention capabilities, which will reduce segregation and water bleeding; (iii) their crimpness (if present) will improve the trixothrophy and flow properties of the wet cementitious matrix, which may find uses in some applications, especially in shotcretes, mortars and grouts; (iv) their low thickness will ensure their higher abundance (number per volume) and will, therefore, bring about better mechanical interactions with the microcracks and, as a result, will provide better cracks prevention performances during settling and therefore better reinforcement during service; and finally (v) due to their lower bending modulus, they will not tend to protrude from the surface of the cementitious composite, as fibers having higher bending modulus such as technical fibers tend to do, and, as such, they will be less prone to harmful environmental conditions (ultraviolet radiation, humidity, heat, etc.) and will bring about better results in some concrete finishing procedures (e.g., Helicopter treatment to smoothen concrete surfaces), wherein the presence of a "beard" is limiting, as the protruding fibers tend to entangle and form small balls.
There is thus a widely recognized need for, and it would be highly advantageous to have, a method to acquire existing textile nylon fibers the required properties and using same for cementitious composites reinforcement. The present invention improves the stability and durability of nylon fibers when used for concrete reinforcement through chemical and thermo mechanical treatments. The mechanical properties of non technical nylon fibers treated according to the teachings of the present invention are improved, as they are strengthened. The nylon fibers of the present invention are superior to any existing fibers of their type in terms of concrete environmental and service stability. The present invention, therefore, facilitates the upgrading of existing standard and low quality nylon fibers rendering same applicable in cementitious composites secondary reinforcement. Thus, upgraded nylon fibers are provided by the present invention for use as superior fibers for secondary reinforcement of concrete and other cementitious composites. The textile nylon fibers according to the present invention are used in the reinforcement of concrete as an alternative to the prior art wire mesh, polypropylene fibers and technical nylon fibers, and are successful in inhibiting shrinkage cracking, increasing impact capacity, reducing permeability, adding shatter resistance and reducing construction time.